# Tag Info

11

For VCA, as the construction is as simple as averaging the potentials of atomic species, the main advantage is computational. This is in the sense that it is very simple to change the compositional ratio of a solid solution by just changing how you average the potentials, instead of having to work with supercells (as you would have to do with SQS or other ...

10

Often we are hung up on the idea of making sure we generate an SQS of a size (i.e. # of atoms) that matches the composition of our system. For instance, in the case of $A_3B$ we would generally take multiples of 4 as the # of atoms. This isn't always enough, especially in your case. Since you want to make a cell that is ordered in certain layers and ...

10

First of all, it's better to define the terms here because probably not all the people here are familiar with SQS right away. SQS is an abbreviation for special quasirandom structures. The illuminating reference here is the work of Zunger et. al. that basically show you that the most relevant parameter to consider when you want if SQS truly resembles a ...

9

Very good question. I think that when you select one of the many different disordered structures as been a representative one, that structure is not disordered anymore, it is an structure as any other. Following this, I think that the convergence test can be done in a similar way that for any other primitive cell. The problem of course, is when you decide ...

5

If you have a system without periodicity like your disordered solid solution, then you should use a $1\times1\times1$ $\mathbf{q}$-point grid for a phonon calculation (equivalent to a $1\times1\times1$ supercell). Using a larger supercell will introduce an artificial periodicity in the system. Having said this, it is still really important to converge with ...

5

For special quasirandom structures, the main advantage is that treating the atoms explictly allows the cell shape and ionic positions to be relaxed, capturing local atomic distortions which play a relatively large role in the electronic structure, energy and other properties. In principle, the VCA and other mean field approaches to disorder capture the same ...

5

I will start by re-stating your question to make sure I understand what you mean. You have a cell with lattice vectors written in Cartesian coordinates as follows:  \begin{pmatrix} \mathbf{a}=\hat{\mathbf{x}}-\hat{\mathbf{y}}-\hat{\mathbf{z}} \\ \mathbf{b}=\hat{\mathbf{x}}+\hat{\mathbf{y}}-\hat{\mathbf{z}} \\ \mathbf{c}=1.5\hat{\mathbf{x}}+2.5\hat{\mathbf{...

5

I used the structure manipulation scripts provided by AIRSS to generate the conventional cell associated with your primitive cell. I think AIRSS uses Spglib under the hood for this task, so you could probably use Spglib directly if you wanted. Original cell New cell POSCAR file POSCAR 1.0000000000000000 10.0886200000 0.0000000000 0.0000000000 0....

4

These are some general comments about modelling disorder which I think address your question, but note I cannot provide specifics about the codes you are using. Imagine a simple configuration with only two sites for the discussion: Both atoms are of the same type (blue) so that this configuration has a mirror plane down the middle of the two atoms (dashed ...

3

There are two different perspectives here. The first one is the creation of the SQS structure itself accomplished through the ATAT package, for example. The second one is the calculation of the slab properties. The SQS procedure returns the smaller structure that resembles the periodic structure properties. For alloys, for instance, the inputs are the ...

2

Caution: Self answer ahead! Let's understand it with an example of a binary $AB$ compound. A correlation function for a particular type of cluster (pair, triangle, tetrahedron, etc.), $\rho$, is defined as $$\rho_\alpha\: =\: <cluster function>_\alpha,\tag{1}$$ Where $\alpha$ represents the type of cluster and $<...>$ ...

1

As Anibal points in his answer, generating the SQS and using it for a surface calculation are two separate things, and shouldn't affect each other. You would still take all precautions as usual while performing the DFT calculation. That being said, there could be scenarios where you may want to restrict/prefer certain atom types amassing at edges or corners ...

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